1. Field of the Invention
The present invention relates to a process for making esters. More particularly, it relates to a continuous process for making fatty acid alkyl esters.
2. Brief Description of Related Art
Biobased products (for example, biodiesel and/or biolubricants) typically include long chain, fatty acid alkyl esters produced from vegetable oils or animal fats by transesterification of the fatty acid glycerides with alcohols. Biodiesel typically include long chain, fatty acid alkyl esters produced from vegetable oils or animal fats by transesterification of the fatty acid glycerides with lower alcohols (for example, methanol and/or ethanol). Biolubricants may be prepared through transesterification of glycerides with alcohols having carbon numbers ranging from 5 to 12 or greater, branched alcohols of similar molecular weight, or transesterification of fatty acid methyl esters. Due to environmental concerns bio-based products in many formulations are being used as substitutes for the petroleum-based products. Biobased products derived from vegetable and plant products, such as soybean, sunflower, and rapeseed etc., are renewable, biodegradable, less environmentally hazardous, and safer to handle. Similarly, other renewable sources of fatty acid glycerides include rendered animal fats and waste cooking oils from commercial food production. Rendered animal fats and waste cooking oils may also be used in the production of biodiesel fuels and biolubricants for automobile applications, mechanical engine applications, cosmetic applications, and soaps.
The heating value of vegetable oil is similar to that of fossil fuel (for example, diesel), but the direct use of vegetable oils in the diesel engines is limited by some of their physical properties. For example, the viscosity of vegetable oil is about 10 times the viscosity of diesel fuel.
Transesterification of fatty acid glycerides may be used to improve the fuel value and lubricant utility of the fatty acid glycerides. The production of useful industrial compounds from naturally-derived and sustainably-produced fatty acid glycerides is made difficult by the presence of lipophilic or oil soluble material which must be removed to permit the following transesterification to reach a high level of conversion and economic efficiency. Such problems with seed oils include degumming, the removal of phospholipids; deodorizing, the removal of free fatty acids; and bleaching, the removal of finely divided solids and colored materials. Conventional processes that use alkaline catalysts for the production of fatty acid methyl esters may be highly sensitive to the presence of contaminates in the fatty acid glyceride phase. Moisture may deactivate the alkaline catalyst. Free fatty acids present in the starting material may inactivate the alkaline catalyst and produce soaps; and unsaponifiable materials may react with such catalysts. Water and/or soaps interfere with the separation of glycerin from the fatty acid alkyl ester mixtures. Additionally, the final product may have to be blended with other oils to adjust the free fatty acid content and/or reduce the content of contaminants in the final product.
Conventional means for recovering valuable glycerin from naturally-derived and sustainably-produced fatty acid glycerides involves saponification of the fatty acid glyceride which affords a crude glycerin product in an aqueous mixture. The crude mixture may or may not include salts and other undesirable materials, requiring expensive dewatering and further refinement.
Processes for the transesterification of fatty alkyl esters using heterogeneous catalysts have been developed. Many of the processes require separation of the catalyst from the system and/or further processing to remove glycerin and/or other by-products of the transesterification process.
U.S. Pat. No. 7,754,643 to Srinivas et al., which is incorporated herein by reference, describes a catalyst and method of use for the transesterification of glycerides, fatty acid esters and cyclic carbonates.
U.S. Pat. No. 7,482,480 to Srinivas et al., which is incorporated herein by reference, describes a process for the preparation of hydrocarbon fuel that includes contacting fatty acid glycerides with alcohols in the presence of a solid, double metal cyanide catalyst at a temperature in the range of 150° to 200° C. for a period of 2-6 hrs and separating the catalyst from the above said reaction mixture to obtain the desired hydrocarbon fuel.
U.S. Pat. No. 7,842,653 to Darbha et al. describes a batch process for the preparation of lubricants from vegetable oil or fat obtained from animal source that involves a reaction of vegetable oil or fat with an alcohol in the presence of a double metal cyanide catalyst, at a temperature in the range of 150° to 200° C. for a period of 3 to 6 hrs to obtain the desired biolubricant.
International Application Publication No. WO/2009/113079 to Srinivas et al., which is incorporated herein by reference, describes a process for the preparation of biofuels or biofuel additives from glycerol.
U.S. Pat. No. 8,124,801 to Srinivas et al., which is incorporated herein by reference, describes a batch process for the preparation of fatty acid alkyl esters using a catalyst that includes a metal from Group VIB of the Periodic Table, a metal from Group IIIA of the Periodic Table and an element group VA of the Periodic Table.
U.S. Patent Application Publication no. 2010/0108523 to Sams et al., which is incorporated herein by reference, describes removal of glycerin from biodiesel using an electrostatic process.
U.S. Pat. No. 7,531,688 to Fleischer, which is incorporated herein by reference, a method for making fatty acid alkyl esters by reacting fatty acid glycerides with an excess of alcohol in a pressurized environment, where the unreacted alcohol component is separated from the reaction product by a flash purification techniques.
Conventional processing (for example, batch processing) to prepare fatty acid alkyl esters, removal of excess alcohols and other volatile compounds is done by reducing a pressure of the reaction vessel and distilling or flashing the excess alcohol from the reaction vessel until all or substantially all of the alcohol is removed from the reaction vessel, which may cause prolonged heating of the reaction mixture in the presence of the alcohol. Prolonged heating of the reaction product may cause thermal degradation of the fatty acid alkyl esters and/or hydrolysis of the fatty acid alkyl esters. Thus, an efficient method of removing alcohols and/or water from the reaction mixture is highly desired.
As described, many methods and/or catalysts for the transesterification of fatty alkyl acids have been proposed, however, many methods require purification of starting materials, removal of water from the starting fatty alkyl acid, and/or steps to remove by-products formed from the esterification reactions. Hence, an efficient method of transesterifying both edible and non-edible vegetable oils in refined or unrefined forms at mild conditions is highly desirable. Moreover, an efficient method of simultaneously converting free fatty acid contaminants of naturally-derived and sustainably-produced fatty acid glycerides at mild conditions is highly desirable. Such combined methods enable economic benefits and make the bioproducts an economical alternative to petroleum based diesel and lubricants.